A quadrupole mass analyzer separates ions with different masses by applying a DC voltage and an rf voltage on four rods having hyperbolic cross sections and an axis equidistant from each rod. Combined DC and rf voltages on the quadrupole rods are set to pass only ions which have a selected mass-to-charge ratio (m/e). All other ions, i.e., those which do not have that selected charge to mass ratio, do not have a stable trajectory through the quadrupole mass analyzer. These other ions will collide with the quadrupole rods, never reaching the detector. The electrode structure 100 required to generate the quadrupole field has hyperbolic cylinders forming electrodes 102-108 with semiaxes both equal to a distance r.sub.o, the so-called field radius. This basic structure is shown in FIG. 1.
A conventional four-rod quadrupole mass analyzer geometry 200 is shown in FIG. 2A. Each of the four rods 202-208 is constrained to a rod radius of approximately 1.147r.sub.o. The whole assembly is typically housed inside a grounded cylinder 210 at radius of approximately 3.54r.sub.o. These values are chosen to make large distorting terms in a quadrupolar electric potential cancel out. The quadrupole mass analyzer may alternately be constructed of 16 rod electrodes in a 4.times.4 array 201 to form nine separate quadrupolar regions 212 as shown in FIG. 2B. This instrument, however, is quite bulky and must be carefully constructed to produce substantially quadrupolar fields. Additionally, the outer cylinder 214 must be constructed so as to shield the inner fields from outside conductors.
The quadrupole mass analyzer has long been one of the most sensitive and transportable instruments for determining the composition of an unknown sample e.g. a gas sample. It has become one of several standard laboratory and commercial instruments for use in chemical analysis, environmental monitoring, and as a residual gas analyzer. The quadrupole mass analyzer is a commonly flown instrument for planetary aeronomy studies. Other uses include planetary surface studies and geological aging.
The quadrupole mass analyzer has probed the earth's atmosphere from aircraft, balloons, and sounding rockets. It has been carried across the solar system as an instrument on the Galileo spacecraft, released into the Jovian atmosphere and has precisely measured the constituents of the atmosphere of this giant planet. The Cassini spacecraft will carry a similar mass analyzer to be dropped into Saturn's upper atmosphere. The long and widespread use of this technology has proven it to be one of the most useful analytical instruments ever developed.
However, in order to fulfill National Aeronautical Space Agency's (NASA) mandate of having "faster, better, cheaper" space missions, smaller instruments are desired in order to reduce mass, volume, and power so that planetary missions are carried out at much less cost and, hence, with a higher frequency.
More recently, ion trap quadrupole mass analyzers have been used to analyze gas samples. In an ion trap analyzer, ions are dynamically stored in a three-dimensional quadrupole ion storage device. The rf and DC potentials are scanned to eject successive mass-to-charge (m/e) ratios from the trap into a detector. In addition, very large masses are stored by reducing the frequency, f of the trapping field since the maximum mass selected is M.sub.max =7.times.10.sup.6 =V.sub.max /(f.sup.2 r.sub.o.sup.2) where V.sub.max is the operating voltage. This typically cannot be achieved in a "single pass" quadrupole mass analyzer mode, especially for a small instrument, since the condition of frequency being much greater than the inverse of ion transit time through quadrupole mass analyzer is necessary for adequate mass resolution.
In a conventional point node trap, electrons enter the trap from outside and must transit through the region of high trap field except during the short time when the phase of the rf is near a zero of the field cycle. The usable portion of the trap, a small volume 4.pi.r.sub.o.sup.3 /3 centered on the node, where the ion creation must occur, is smaller than the similar volume, .pi.r.sub.o.sup.2 L, within a distance r.sub.o of the node line in linear trap of length L. This is so because L is significantly larger than r.sub.o.